Integrated circuits in a semiconductor device are comprised of metal wiring that is insulated by a dielectric layer to prevent capacitance coupling or crosstalk between the electrical pathways. Metal wiring that forms interlevel and intralevel connections which are commonly referred to as interconnects is frequently formed by depositing a metal in an opening such as a via hole or a trench in a single damascene approach or in a trench and via simultaneously in a dual damascene scheme. Usually, a diffusion barrier layer is formed on the sidewalls of the dielectric layer in the via and/or trench before metal deposition to protect the metal from corrosion and oxidation and to prevent metal ions from migrating into the dielectric layer. The metal layer is typically planarized by a chemical mechanical polish (CMP) process that removes excess metal above the dielectric layer so that the metal layer becomes coplanar with the dielectric layer.
Recent progress in forming metal interconnects includes lowering the resistivity (R) of the metal by replacing aluminum with copper, decreasing the width of the vias and trenches with improved lithographic materials and processes, and reducing the dielectric constant (k) of the dielectric layer to minimize capacitance coupling (c) between the metal interconnects. Thus, by minimizing R and c simultaneously, the speed of an integrated circuit becomes faster because of a lower Rc delay.
As the widths of vias and trenches shrink, the semiconductor industry is faced with the increasingly difficult challenge of forming a copper interconnect without voids. Voids can easily form along grain boundaries during metal deposition and cause reliability issues. In addition, a premium is placed on metal purity in the interconnect in order to ensure high performance and reliability. For instance, copper is typically deposited by an electroless or electroplating technique that involves an electrolyte solution containing sulfate and chloride ions. Small amounts of these ions are frequently trapped inside the deposited copper layer and the resulting sulfate and chloride impurity can lead to corrosion and reduced performance. Furthermore, organic additives containing carbon are frequently used in the electrolyte solution and carbon may become trapped within the deposited metal layer. Therefore, a copper deposition method is needed that minimizes impurities within an interconnect.
Referring to FIG. 1, a conventional copper damascene structure is illustrated. A substrate 1 is shown having a conductive layer 2 that has an exposed top surface. An etch stop layer 3 and a dielectric layer 4 are successively formed on substrate 1. A via opening 5 is formed by a well known photoresist patterning (not shown) and plasma etch sequence and is aligned above the conductive layer 2. Next, a trench 6 is formed above the via and the etch stop layer 3 above the conductive layer 2 is removed. A conformal diffusion barrier layer 7 is deposited on the sidewalls and bottom of the via 5 and trench 6 followed by performing an electroless or electroplating method, for example, to fill the via and trench with a copper layer 8. The copper damascene process is completed by a planarization step that typically involves a chemical mechanical polish (CMP) process in which the copper layer 8 becomes coplanar with the dielectric layer 4.
Referring to FIG. 2, a portion of the copper layer 8 in FIG. 1 is shown which has a grain boundary 8a. A void defect 9 is shown along the grain boundary 8a. Unfortunately, in a prior art process where only one anneal is performed after the copper layer 8 is deposited, there is an unacceptably large number of void defects formed in the copper layer, especially along a grain boundary. This problem is believed to result from impurities trapped within the copper layer 8 and in particular carbon impurities that are caused by using organic additives in the copper deposition process. Since the copper layer 8 may be several thousand Angstroms thick, a single anneal at the end of the copper deposition process is not capable of removing carbon impurities in the lower portion of the copper layer.
In U.S. Pat. No. 6,077,780, a copper seed layer is deposited in an opening and is annealed above 200° C. to reflow the copper seed layer and minimize the number of seams or grain boundaries in the layer. Then an electroless or electroplating step is used to deposit a second copper layer that fills the opening. However, this method does not address the need to reduce impurities in the second copper layer.
In U.S. Pat. No. 6,245,670, a via in a dual damascene structure is filled by an electroless plating process. A barrier layer and a copper seed layer are sequentially deposited within the trench which is then filled by an electroplating process. The method is designed to enable a more effective fill of the via and trench but does not teach how to reduce C, S, or Cl impurities.
A two stage copper anneal to improve reliability of a copper damascene interconnect is disclosed in U.S. Pat. No. 6,391,777. A first anneal is performed after a copper layer is deposited in a via and trench and a second anneal at a higher temperature is carried out following a CMP process to planarize the copper layer.
In U.S. Pat. No. 6,380,084, a via is formed in a first dielectric layer and filled with a first copper layer before a trench is formed in a second dielectric layer above the via and is filled with a second copper layer. This method does not address the impurity concern and requires additional process steps which add to manufacturing cost.
A self ionized plasma (SIP) method for sputtering copper is described in U.S. Pat. No. 6,582,569. The SIP layer serves as a seed or nucleation layer in high aspect holes.
In patent application publication U.S. 2003/0140988, a catalytic layer of a noble or semi-noble metal is deposited on a seed layer to patch any discontinuities. A conductive layer such as copper is deposited on the catalytic layer and may be annealed by two step process. However, both annealing steps occur after the copper deposition which does not completely remove impurities from a thick copper layer.
Therefore, an improved method of forming copper interconnects is needed which affords good trench and via filling capability to minimize voids while providing a pathway for higher metal purity even in thick copper layers.